Thermal printing is a term broadly used to describe several different families of technology for making an image on a substrate. Those technologies include hot stamping, direct thermal printing, dye diffusion printing and thermal mass transfer printing.
Hot stamping is a mechanical printing system in which a pattern is stamped or embossed through a ribbon onto a substrate, such as disclosed in U.S. Pat. No. 4,992,129 (Sasaki et al.). The pattern is imprinted onto the substrate by the application of heat and pressure to the pattern. A colored material on the ribbon, such as a dye or ink, is thereby transferred to the substrate where the pattern has been applied. The substrate can be preheated prior to imprinting the pattern on the substrate. Since the stamp pattern is fixed, hot stamping cannot easily be used to apply variable indicia or images on the substrate. Consequently, hot stamping is typically not useful for printing variable information, such as printing sheets used to make license plates.
Direct thermal printing was commonly used in older style facsimile machines. Those systems required a special substrate that includes a colorant so that localized heat can change the color of the paper in the specified location. In operation, the substrate is conveyed past an arrangement of tiny individual heating elements, or pixels, that selectively heat (or not heat) the substrate. Wherever the pixels heat the substrate, the substrate changes color. By coordinating the heating action of the pixels, images such as letters and numbers can form on the substrate. However, the substrate can change color unintentionally such as when exposed to light, heat or mechanical forces.
Dye diffusion thermal transfer involves the transport of dye by the physical process of diffusion from a dye donor layer into a dye receiving substrate. Similar to direct thermal printing, the ribbon containing the dye and the substrate is conveyed past an arrangement of heating elements (pixels) that selectively heat the ribbon. Wherever the pixels heat the ribbon, solid dye liquefies and transfers to the substrate via diffusion. Some known dyes chemically interact with the substrate after being transferred by dye diffusion. Color formation in the substrate may depend on a chemical reaction. Consequently, the color density may not fully develop if the thermal energy (the temperature attained or the time elapsed) is to low. Thus, color development using dye diffusion is often augmented by a post-printing step such as thermal fusing. Alternatively, U.S. Pat. No. 5,553,951 (Simpson et al.) discloses one or more upstream or downstream temperature controlled rollers to provide greater temperature control of the substrate during the printing process.
Thermal mass transfer printing, also known as thermal transfer printing, non-impact printing, thermal graphic printing and thermography, has become popular and commercial successful for forming characters on a substrate. Like hot stamping, heat and pressure are used to transfer an image from a ribbon onto a substrate. Like direct thermal printing and dye diffusion printing, pixel heaters selectively heat the ribbon to transfer the colorant to the substrate. However, the colorant on the ribbon used for thermal mass transfer printing includes a polymeric binder, typically composed of wax and/or resin. Thus, when the pixel heater heats the ribbon, the wax and resin mass transfers from the ribbon to the substrate.
One problem with thermal mass transfer printing is producing high quality printing on non-compatible surfaces, such as non-planar or rough surfaces, surfaces with non-uniform thermal conductivity, and when the composition of the substrate is not chemically compatible with the binders in the colorant.
FIG. 1 illustrates one example of a substrate 20 that has both a rough or non-smooth printing surface 22 and a non-homogenous thermal conductivity. The retroreflective sheeting 20 includes a plurality of glass beads 24 attached to a backing 26 by resin/polymer matrix 28. In the illustrated embodiment, a retroreflective layer 29 is interposed between the backing 26 and the resin/polymer matrix 28. The glass beads 24 protrude from the resin/polymer matrix 28 typically by an amount of about 1 micrometers to about 5 micrometers, forming a rough or non-planar surface for thermal mass transfer printing.
Since the retroreflective sheeting 20 is not constructed of a single, homogenous material, the thermal conductivity along the printing surface 22 may vary. For example, the thermal conductivity of the glass beads 24 may be different from thermal conductivity of the resin/polymer matrix 28. In addition, thermal conductivity may be effected by the varying thickness of the backing 26, voids in the backing 26 or mounds or piles of glass beads 24 on the retroreflective sheeting 20. Consequently, applying an image to the printing surface 22 using conventional thermal mass transfer printing techniques can result in a variable thickness in the thermal mass transfer layer 23 and/or a variable adhesion of the colorant pixel dots, with a corresponding degradation in the print quality.
FIG. 2 illustrates an alternate substrate having a printing surface 30 with variable thermal conductivity. FIG. 2 illustrates a sealed or encapsulated retroreflective sheeting 32. Microspheres or glass beads 34 are bonded to a bonding layer 36 with an optional reflecting layer 38 interposed therebetween. A protective layer 40 is attached to the bonding layer 36 by a plurality of raised supports 42. The protective layer 40 forms a space 44 above the microspheres 34. Consequently, the thermal conductivity of the printing surface 30 varies significantly between the regions over the spaces 44 and regions over the raised supports 42. It is typical for the thickness and percent coverage of a thermal mass transfer layer 46 to vary between the regions over the spaces 44 and the regions over the raised supports 42.
FIG. 3 illustrates an example of sealed or encapsulated retroreflective sheeting in which the raised supports form a hexagonal pattern on the printing surface. Due to the variation in thermal conductivity of the printing surface, the hexagonal pattern of the raised support shows through the printed image on the retroreflective sheeting of FIG. 3.
U.S. Pat. No. 5,818,492 (Look) and U.S. Pat. No. 5,508,105 (Orensteen et al.) teach that thermal mass transfer printing can be performed on retroreflective sheeting in those instances where there is a polymeric layer or layers disposed thereon. While adding a polymeric layer has improved printability on some retroreflective sheeting, the process of adding the layer increases the cost of the final product and can degrade the retroreflective properties of the substrate. Even with the additional layer, the print quality is inadequate for some graphics applications. Adding a printable layer may alter other characteristics of the retroreflective sheeting, such as frangibility.
In order to use thermal mass transfer printing on a non-compatible surface, the most common methods of improving print quality is to increase the thermal energy of the print head and to increase the pressure applied to the print head by the backup roll. However, increasing thermal energy and pressure can lead to decreased printer head life, ribbon wrinkling, lower print quality and mechanical stresses in the printing system. Therefore, what is needed is a method and apparatus for thermal mass transfer printing on substrates that have a rough surface, non-homogenous thermal conductivity, and/or a surface composition that is not immediately compatible with the colorant of the thermal mass transfer printing ribbon.